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Cluster – new insights into the electric circuits of polar lights

9 February 2007

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Auroras form in high latitude regions of Earth, and appear in many different shapes. The aurora in the early evening sky forms a green arc that stretches across the sky in an east-west direction. The longitudinal extent (length) of an auroral arc can be as large as several thousands kilometres, but its width can be as small as 100 metres.

The deep mechanisms that rule the creation of such beutiful natural light displays (also called polar lights), have been the subject of studies that are keeping solar and plasma scientists busy since years.

Credits: Jan Curtis, Fairbanks, Alaska

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In an old television tube (not the recent LCD or plasma TV screens), accelerated electrons hit a phosphorescent screen and cause the phosphor to glow. The acceleration region generating the aurora works similarly; electrons are accelerated on their way down to the ionosphere. After being accelerated, they finally crash into ionospheric atoms and molecules and cause them to glow, creating aurorae.

Credits: NASA/ESA

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Spiralling-down electrons create auroras

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Artistic view of electrons, responsible for aurora, spiralling down magnetic field lines. The U-shaped potential structure illustrates the region where electrons get accelerated on their way down to the upper atmosphere. Here they are stopped by collisions with neutral atoms and molecules, primarily oxygen and nitrogen, at altitudes of a few hundreds kilometres down to 80 kilometres. Each collision transfers part of the electron energy to these atmospheric particles. In turn, they get rid of this energy excess by emitting visible emissions in specific wavelength (or colours) such as green (oxygen) or purple (nitrogen).

Credits: ESA

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Left side. Auroral oval (in false colour) as seen from space, overlaid on top of a visible image of Earth. The red indicates the brightest aurora and blue the dimmest. The brightest aurora is found at midnight.

Right side. Locations of 4 auroral oval crossings by the Cluster spacecraft projecting down to the ionosphere with respect to nightside auroral plasma boundaries. In this projection the Earth is viewed from above the North pole and polar cap, the Sun is located at the top of the image. Events 1 and 2 from early 2001 were both located at the boundary between the Central Plasma Sheet (CPS, in green) and the Plasma Sheet Boundary Layer (PSBL, in blue). Events 3 and 4 in 2002 were found at the boundary between the PSBL and the polar cap.

Credits: Left: Holzworth/Meng, NSSDC, NASA. Right: Marklund/Alfvén Lab.

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